Dielectric resonators are used in many circuits, particularly microwave circuits, for concentrating electric fields. They can be used to form filters, oscillators, triplexers, and other circuits. The higher the dielectric constant of the dielectric material out of which the resonator is formed, the smaller the space within which the electric fields are concentrated. Suitable dielectric materials for fabricating dielectric resonators are available today with dielectric constants ranging from approximately 10 to approximately 150 (relative to air). These dielectric materials generally have a mu (magnetic constant, often represented as μ) of 1, i.e., they are transparent to magnetic fields.
FIG. 1 is a perspective view of a typical cylindrical or doughnut-type dielectric resonator of the prior art that can be used to build dielectric resonator circuits such as filters. As can be seen, the resonator 10 is formed as a cylinder 12 of dielectric material with a circular, longitudinal through hole 14. Individual resonators are commonly called “pucks” in the relevant trade. While dielectric resonators have many uses, their primary use is in connection with microwaves and particularly, in microwave communication systems and networks.
As is well known in the art, dielectric resonators and resonator filters have multiple modes of electrical fields and magnetic fields concentrated at different center frequencies. A mode is a field configuration corresponding to a resonant frequency of the system, as determined by Maxwell's equations. In a typical dielectric resonator circuit, the fundamental resonant mode, i.e., the field having the lowest frequency, is the transverse electric field mode, TE01 (or TE, hereafter). The electric field 31 of the TE mode is circular and is oriented transverse of the cylindrical puck 12. It is concentrated around the circumference of the resonator 10, with some of the field inside the resonator and some of the field outside the resonator. A portion of the field should be outside the resonator for purposes of coupling between the resonator and other microwave devices (e.g., other resonators or input/output couplers) in a dielectric resonator circuit.
It is possible to arrange circuit components so that a mode different than the TE mode is the fundamental mode of the circuit and this, in fact, is done sometimes in dielectric resonator circuits. Also, while typical, there is no requirement that the fundamental mode be used as the operational mode of a circuit, e.g., the mode within which the information in a communications circuit is contained.
The second mode (i.e., the mode having the second lowest frequency) normally is the hybrid mode, H11 (or H11 mode hereafter). The next lowest-frequency mode usually is the transverse magnetic (or TM) mode. There are additional higher order modes. Typically, all of the modes other than the fundamental mode, e.g., the TE mode, are undesired and constitute interference. The H11 mode, however, typically is the only interference mode of significant concern, particularly during tuning of dielectric resonator circuits. However, the transverse Magnetic TM mode sometimes also can interfere with the TE mode. The remaining modes usually have substantial frequency separation from the TE mode and thus do not cause significant interference with operation of the system. The H11 mode, however, tends to be rather close in frequency to the TE mode and thus can be difficult to distinguish from the TE mode in operation. In addition, as the frequency and bandwidth (which is largely dictated by the coupling between electrically adjacent dielectric resonators) of the TE mode is tuned, the center frequency of the TE mode and the H11 mode move in opposite directions to each other. Thus, as the TE mode is tuned to increase its center frequency, the center frequency of the H11 mode inherently moves downward and, thus, closer to the TE mode center frequency.
FIG. 2 is a perspective view of a microwave dielectric resonator filter 20 of the prior art employing a plurality of dielectric resonators 10. The resonators 10 are arranged in the cavity 22 of an enclosure 24. Microwave energy is introduced into the cavity via a coupler 28 coupled to a cable, such as a coaxial cable. Conductive separating walls 32 separate the resonators from each other and block (partially or wholly) coupling between physically adjacent resonators 10. Particularly, irises 30 in walls 32 control the coupling between adjacent resonators 10. Walls without irises generally prevent any coupling between adjacent resonators. Walls with irises allow some coupling between adjacent resonators. By way of example, the field of resonator 10a couples to the field of resonator 10b through iris 30a, the field of resonator 10b further couples to the field of resonator 10c through iris 30b, and the field of resonator 10c further couples to the field of resonator 10d through iris 30c. Wall 32a, which does not have an iris, prevents the field of resonator 10a from coupling with physically adjacent resonator 10d on the other side of the wall 32a. Conductive adjusting screws may be placed in the irises to further affect the coupling between the fields of the resonators and provide adjustability of the coupling between the resonators, but are not shown in the example of FIG. 2.
One or more metal plates 42 may be attached by screws 43 to the top wall (not shown for purposes of clarity) of the enclosure to affect the field of the resonator and help set the center frequency of the filter. Particularly, screws 43 may be rotated to vary the spacing between the plate 42 and the resonator 10 to adjust the center frequency of the resonator. An output coupler 40 is positioned adjacent the last resonator 10d to couple the microwave energy out of the filter 20 and into a coaxial connector (not shown). Signals also may be coupled into and out of a dielectric resonator circuit by other methods, such as microstrips positioned on the bottom surface 44 of the enclosure 24 adjacent the resonators. The sizes of the resonator pucks 10, their relative spacing, the number of pucks, the size of the cavity 22, and the size of the irises 30 all need to be precisely controlled to set the desired center frequency of the filter and the bandwidth of the filter. More specifically, the bandwidth of the filter is controlled primarily by the amount of coupling of the electric and magnetic fields between the electrically adjacent resonators. Generally, the closer the resonators are to each other, the more coupling between them and the wider the bandwidth of the filter. On the other hand, the center frequency of the filter is controlled largely by the size of the resonators themselves and the size of the conductive plates 42 as well as the distance of the plates 42 from their corresponding resonators 10. Generally, as the resonator gets larger, its center frequency gets lower.
Prior art resonators and the circuits made from them have many drawbacks. For instance, prior art dielectric resonator circuits such as the filter shown in FIG. 2 suffer from poor quality factor, Q, due to the presence of many separating walls and coupling screws. Q essentially is an efficiency rating of the system and, more particularly, is the ratio of stored energy to lost energy in the system. The fields generated by the resonators pass through all of the conductive components of the system, such as the enclosure 20, plates 42, internal walls 32 and 34, and adjusting screws 43, and inherently generate currents in those conductive elements. Those currents essentially comprise energy that is lost to the circuit.
Furthermore, the volume and configuration of the conductive enclosure 24 substantially affects the operation of the system. The enclosure minimizes radiative loss. However, it also has a substantial effect on the center frequency of the TE mode. Accordingly, not only must the enclosure usually be constructed of a conductive material, but also it must be very precisely machined to achieve the desired center frequency performance, thus adding complexity and expense to the fabrication of the system. Even with very precise machining, the design can easily be marginal and fail specification.
Even further, prior art resonators tend to have poor mode separation between the TE mode and the H11 mode.
Accordingly, it is an object of the present invention to provide improved dielectric resonators.
It is another object of the present invention to provide improved dielectric resonator circuits.
It is a further object of the present invention to provide dielectric resonator circuits with improved quality factor, Q.